Atomic Number
Chemistry: Edexcel IGCSE, topic 3.1.4
Atomic Number and Mass Number
The atomic number is the number of protons in the atom. The mass number of an atom (or relative mass) is calculated by adding the number of protons and neutrons found in a nucleus. These numbers can be found for each element in the periodic table.
Sodium
- Mass number (top) = 23.
- Atomic number (bottom) = 11.
Carbon
- Mass number (top) = 12.
- Atomic number (bottom) = 6.
Test yourself on Atomic Number
Which determines an element's atomic number?
Magnesium (Mg) has an atomic number of 12. How many protons and electrons does a neutral Mg atom have?